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  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
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    <!-- 定义一个变量a，使得a == 1 && a == 2 && a == 3 的
表达式结果为true -->
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  <script>
    const a = 1;
    console.log(/^(1|2|3)$/.test(a)); // true
  </script>
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